Ion-mediated signaling plays a controlling role in nearly all biological processes. Presently, the patch clamp technique is the primary means of studying localized voltage-gated events in live cells. However, a significant level of expertise is required to make reliable patch clamp measurements. New techniques for cell-stimulation are in demand in the cell signaling community.
There is widespread interest in voltage-gated cellular signaling mechanisms and their associated processes and diseases. These include migraine pain, wound-healing, and possibly even metastatic disease. However, investigating such processes on the single cellular and sub-cellular levels is challenging, so new techniques for cell-stimulation are in high demand.
A large number of the processes that cells and tissue undergo are mediated by trans-membrane and intracellular ion-fluxes. Dictyostelium cells are of interest to the voltage-gated signaling community, in part because they are model systems for studying electric field-induced migration of the cells. In mammals, this process plays an important role in wound healing and tissue regeneration. Electric fields arise naturally in traumatized tissue and have been shown to induce the migration of human keratinocytes and corneal epithelial cells along the field gradients. Disruption of these fields impairs wound-healing. It has also been found that a reduction of the transepithelial potential in cancerous rat prostates promotes the invasion of the surrounding tissue by metastatic cells. The amoeboid Dictyostelium relates to these mammalian processes because Dictyostelium exhibits strong electrotactical behavior that is similar to that of many other cell types. It is also genetically tractable so the effects of knocking out particular receptors, chemoattractants, and channels may be characterized. Finally, Dictyostelium is convenient to work with because it grows well at room temperature in phosphate buffer.
Recent work on the voltage stimulated behavior of Dictyostelium cells has been performed by placing macroscopic electrodes adjacent to cultures in order to induce migration. A 2006 report by Shanley and co-workers showed that this approach induced all of the cells to move with roughly similar rates towards the cathode. The electrode dimensions were much larger than the cells, so the extent to which these crowded populations were responding independently to the applied field or whether a significant degree of cell-to-cell signaling played a part in this phenomenon was not clear. Stimulating single cells, as opposed to applying the voltage across the entire sample, would resolve more detail of such cellular behavior.
The difficulty of realizinig single-step growth and interconnecting of diameter tunable nanowires is a widely recognized problem in the nanotechnology community. A number of fabrication techniques provide control over the nanowire diameter. The vapor-liquid-solid method uses metallic nanodroplets to catalyze the condensation, nucleation, and axial growth of vaporous growth-material to produce pristine arrays of near single crystalline nanowires from a wide variety of materials. The size of the catalytic nanodroplets dictates the diameter of the nanowires, which can be as small as 1 nm, and influences the crystallographic direction in which the wires grow. In another approach, porous substrates, nanotubes, DNA, and other biomolecules are used as templates for the formation of nanowires with very small diameters and a wide range of intricate shapes. Here, the nanowire diameter is determined by the pore size of the template and can extend from microns down to the sub-nanometer scale. Other templating techniques use selectively etched substrates to control wire growth, enabling the fabrication of metallic nanowire arrays with sub-20 nm wire diameters and wire-to-wire separations. A fourth technique uses ultrasonic stimulation of simple salt and sugar solutions which induces the growth of metallic nanobelts. In this approach, the width of the nanobelts ranges from 8 nm to 20 nm and is controlled by the duration of the ultrasonic irradiation. With these techniques, connecting the individual wires with external instrumentation and with other submicron components is accomplished by secondary processing steps that follow the wire synthesis.
The classic approach to delivering electrical stimuli to a targeted site on a live cell is the patch-clamp technique. However, a significant level of expertise is required to perform reliable patch clamp measurements, so new techniques for cell stimulation are in demand in the cell signaling community. To this end, Lieber and co-workers have cultured nerve cells onto nanoscale electrode arrays, in order to realize multi-electrode stimulation of an individual nerve cell. However, further improvements in the ability to make reliable electrical and signaling contacts with living cells and tissues find broad applications in many areas.
Dielectrophoretic nanowire assembly exploits the voltage-induced chaining and fusing of nanoparticles into wires that span the gaps between opposing electrodes; thereby, the wire assembly and the electrode-wire contacts are accomplished in a single step. Using this technique, wires may now be grown between targeted points on the two electrodes. The transport properties of gold nanoparticle-based dielectrophoretic wires have been shown to have good reproducibility. However, the resistivity of this material is ˜2000 μΩ-cm, three orders of magnitude greater than that of bulk gold. The resistive nature of these wires is due in large part to their particulate structure, as evidenced by the occurrence of the Coulomb blockade at reduced temperatures. While such materials are needed for devices like variable capacitors, the directed growth of more highly conductive, metallic wires is of obvious importance in nanoelectrictronics.
What is needed is a method for addressing the above and related issues.